Sensors for the measurement of position/displacement/proximity, may use resistive, capacitive, inductive or optical methods. Position/displacement sensors measure the linear or angular position and are typically connected mechanically between the point or object being sensed and a reference or fixed point or object.
Variable resistors use a mechanical linkage to connect the point or object being sensed to a wiper or moveable arm that slides over the resistance element. Capacitor sensors are generally used for linear rather than angular measurements wherein the dielectric or one of the capacitor plates is connected to the point or object that is moved for displacement measurement. A non-contacting capacitive position sensor is illustrated in Skalski, "Capacitance Distance Transducer," Proceedings of the IEEE, Vol. 56, No. 1, January 1968, pp. 111-112.
Inductive sensors may comprise single-coil units which use a change in the self-inductance of the coil or multiple-coil units which rely on the change in magnetic coupling or reluctance between coils. Single-coil displacement sensors use a moveable core connected to the measured object to change the self-inductance whereas single-coil proximity sensors use the magnetic properties of the object itself to modify the self-inductance. The change in inductance is usually sensed with an oscillator-driven bridge circuit.
Multiple-coil inductive sensors typically comprise the differential transformer and its variations. The linear variable differential transformer (LVDT) uses three windings and moveable core to sense linear displacement. A typical LVDT has a moveable core which is coaxial with the windings of both an AC input coil and secondary windings on either side thereof. The secondaries are wound to produce opposing voltages and are connected in series. With the core in a neutral or zero position, voltages induced in the secondary windings are equal and opposite and the net output is a minimum. Displacement of the core increases the magnetic coupling between the primary coil and one of the secondary coils and decreases the coupling between the primary coil and the other secondary coil. The net voltage increases as the core is moved away from the center position and the phase angle increases or decreases as a function of the direction in which the core is moved.
A demodulator circuit can be used to produce a DC output from this winding configuration. Differential transformers are also available for angular measurement in which the core rotates about a fixed axis.
The output from such a displacement sensor may be an analog or digital function of the absolute distance being sensed or it may be a function of the distance from a given starting point. U.S. Pat. No. 4,651,130 shows how to inexpensively convert analog-sensed information from an LVDT or RVDT into digital form. Sometimes there is a requirement to measure linear or angular motion without any mechanical linkage, however.
In terms of technical and economic requirements, potentiometer transducers are simple to apply and can be used with very high output levels, e.g., 50 V or higher and for displacements up to half a meter or from 5.degree. to 3600.degree. but must be mechanically linked to the object or point to be measured. Reluctive transducers, on the other hand, with DC-to-DC conversion circuitry offer displacements between 0.25 millimeter and 3 meter and between 0.05.degree. and 90.degree. and do not necessarily have to be mechanically linked to the object being measured. In AC systems multiple-coil inductive sensors are used more than all others.
UK Pat. Spec. No. 1079894 discloses a proximity detection device for an elevator cage. The device includes induction coils N.sub.1, N.sub.2 and N.sub.3 mounted on the cage to sense magnetic shield plates B mounted at strategic locations in the elevator shaft. See FIGS. 1-3d and lines 29-58 of page 3.
A displacement sensor example is shown in U.S. Pat. No. 3,205,485 to Noltingk which, however, discloses relative transverse movement only (constant gap) between a magnetic or non-magnetic tapered screening vane (A) attached to a slide (c) object and one or more primary/secondary pairs(s) which may use pot-shaped cores with an inductance of about 1 mH and having outside dimensions of about 16 mm with a central portion 6 mm in diameter having 200 turns, with the primary coil excited with 8 volts at 24 kHz with the signal across the secondary coils about 0.68 volt/mm movement of the screen.
In Noltingk, the screen moves perpendicularly to the axis of the primary/secondary. The obtruding of the screen to a greater or lesser extent into the field between the output and pickup coil alters the magnitude of the signal induced in the pickup coil. The disclosure points out that this is distinct from the movement of a ferromagnetic member in the field of an inductance, where the latter operates primarily to reduce the reluctance of one or more magnetic circuits, and its presence increases the flux linking two or more inductances. When a screen is interposed between two inductances, on the other hand, their coupling is reduced because of the magnetic shunting effect, and also as a consequence of the field set up by the currents induced in the screen opposing the field generated by that inductance through which a current is passed. In Noltingk, as in Schulz, there is no disclosure of movement of the screen along the primary/secondary axis. Noltingk shows that either the coil assembly or the screen can be stationary, while the other moves.
In the Noltingk patent, the difference between the prior art and the method disclosed by Noltingk is shown by the fact that with the Noltingk invention, a non-ferromagnetic member can be used as a screen and which approach, has been found to increase sensitivity considerably. The inventor claims sensitivity of 10,000:1.
Another contact method is shown in Zabler (U.S. Pat. No. 4,649,340) where a magnetic differential position sensor is illustrated with coil windings in FIG. 4a on the outer arms of an E-shaped core arrangement, while FIGS. 5a and 6 show primary windings on the outer arms and a sensing winding on the inner arm of the E-shaped core. The coils can slide along the arms and are attached to the object being measured. See column 6, lines 5-19 and lines 39-60. Lines 61-68 of column 6 of Zabler (U.S. Pat. No. 4,649,340) suggest moving the core element instead of the object.
A non-contact method is shown in Widdowson et al (U.S. Pat. No. 3,890,516) where the use of E-cores is illustrated with coil windings that sense the position of non-magnetic conductive areas of a track on a rotating drum to provide timing pulses for engine ignition. The windings 24 and 25 on core 22, as shown in FIG. 2, produce an output signal that is processed by the circuitry of FIGS. 1 and 4. The gap between the rotating drum and E-cores is constant.
U.S. Pat. No. 3,961,243 to Schulz shows another non-contact magnetic sensor, in FIG. 2, U-shaped magnetic cores 11 and 12, with windings on one of the legs, between which a movable magnetic armature 10 is disposed for axial movement therebetween. Transverse movement is not disclosed. See column 3, lines 22-30. FIG. 4 again shows only axial movement of a core within a pair of coaxial coils.
Ando (U.S. Pat. No. 4,754,849) shows non-contacting gap detectors 33, 34 and 35 mounted on an elevator passenger cage for gap control purposes. The detectors may be of the electromagnetic type. See FIGS. 5-8 and column 3, lines 18-22. Presumably, due to side-to-side and front-to-back movements of the elevator car, the detectors can move in both dimensions of the illustrated plane with respect to the hoistway rail, as well as vertically.
U.S. Pat. No. 5,294,757 to Skalski et al shows a one-dimensional non-contacting sensor and shows in FIG. 43a a pair of position sensors 1376, 1378, responsive to the position (POS) of the cab shown in FIG. 44, one sensor of which has a response as shown in FIG. 45, and the combination of which forms the composite response of FIG. 46.
U.S. Pat. No. 5,329,077 to Skalski et al discloses an elevator control system which utilizes sensors for storing horizontal deviation of the rail's surface. FIGS. 8 and 9 include GAP sensors 158 and 162. See column 6, lines 10-41. In column 7, lines 1-17, the use of an LVDT or two separate LVDTs is suggested.
When a differential transducer is used it is important to maintain a constant scale factor throughout the range and under differing conditions. For instance, U.S. Pat. No 3,079,545 discloses an LVDT measurement device that is stabilized against various circuit variations, such as temperature, frequency changes, etc., by comparing the sum of the secondary voltages, which theoretically should be constant to a stable reference signal source, and using any error obtained in the comparison to change the excitation of the primary, thereby creating a feedback loop for keeping the excitation as seen by the secondaries constant. There is a showing of a coaxial arrangement of the secondaries and primary. The physical nature of the primary and secondary windings is not disclosed and may be assumed to be conventional.
U.S. Pat. No. 4,904,921 to DeVito et al shows an LVDT position transducer which includes an interface circuit for producing an output signal that is representative of the difference of the secondary winding signals divided by their sums. See column 3, lines 1-10. This equation computes the movable core position. However, it seems to imply the scale factor is constant, although it also states that the decoder provides excellent scale factor stability and linearity and is relatively insensitive to variations in primary drive amplitude. This may mean that the scale factor K may be assumed to be constant, which assumption may be valid, given the claimed excellent stability thereof.
FIG. 2 of U.S. Pat. No. 4,982,156 to Lewis et al shows an E-shaped core 74 for an RVDT displacement transducer. See column 3, lines 14-30. A CPU 70 calculates the position using a ratio (A-B)/(A+B). See column 4, lines 50-53.
U.S. Pat. No. 4,591,795 to McCorkle shows a ratiometric technique in a LVDT or RVDT, but only operating in a unipolar mode.
U.S. Pat. No. 4,387,339 to Akerblom discusses (column 1, lines 12-20) measurement of the space between two discs of magnetic material by measuring the reluctance in an electrical circuit by means of an inductive position indicator disposed in one of the discs. This method had the problem of iron losses causing temperature dependence, etc. The invention provides a feedback circuit that zeroes a measuring element 30 on a central core of an E or pot-shaped sensor 14 by changing the current to the coils 22, 24 on either side of the measuring element. A measure of the spacing between a surface 10 and a surface 12 is thereby provided. A measuring device 36 measures the difference between the current generators 34, 28.
U.S. Pat. No. 3,336,525 to Church shows a two-coil embodiment of a pressure transducer in FIG. 2 with a non-magnetic diaphragm in between two coils 36, 38 in respective pressure chambers. A differential pressure between the chambers causes the diaphragm to move closer to one of the coils, thereby increasing eddy current losses in that coil. The coils are wired into a balanced bridge, so that this eddy-current difference can be picked up and translated into pressure differential by way of the positional change of the diaphragm. FIGS. 12 and 14-16 show a single sensor head 80 embodiment where the two coils 82, 84 are in the same head and wired in a bridge circuit shown in FIG. 17 for sensing the distance between the sensor head 80 and a plate 109. FIG. 18 shows the result in curve No. 3, which has a linear range, as shown.